1. Field of the Invention
The present invention relates to a production method of a heat-treated steel member.
2. Description of Related Art
Some steel members are used in an as rolled and not heat-treated state, but other steel members requiring a high wear resistance (hardness), tensile strength and toughness are heat-treated after rolling,. Such a steel member that is heat-treated and then used is called as a heat-treated member. Typical heat-treated members include a shoe, a link, a pin and a bushing used for an endless track mounted to construction vehicles such as a power shovel and a bulldozer, and a cutting edge of a blade used for a bulldozer and a vehicle with snow plough. Further, many other structural components used in various kinds of industrial machines are made from the heat-treated members.
As illustrated in FIG. 2, a conventional production method of a heat-treated member includes a shaping step 11 in which a steel material is shaped into a predetermined configuration to be a shaped material, and a heat treatment step 12 in which the shaped material is heat-treated. The heat treatment step 12 involves two steps of quench-hardening and tempering, which are typically both performed (Japanese Patent Publication No. HEI 3-219043).
The reason why the heat treatment step requires two steps, i.e., quench-hardening and tempering, is as follows:
Generally, with the heat-treated member, the characteristics of wear resistance (hardness), and tensile strength and toughness are incompatible characteristics with each other. More particularly, in a case where quench-hardening only is conducted, wear resistance (hardness) and tensile strength are greatly improved, while toughness is remarkably low. In the case where the heat treatment includes quench-hardening followed by tempering, although wear resistance (hardness) and tensile strength are slightly lowered, toughness is greatly improved so that necessary wear resistance (hardness), tensile strength and toughness are ensured. In other words, in a case where both quench-hardening and tempering are not conducted, the necessary wear resistance (hardness), tensile strength and toughness are not ensured.
However, the conventional production method has the following problems:
First, since the heat treatment requires two steps, i.e., quench-hardening and tempering, the number of production steps, production time and equipments are increased, resulting in an increase in the production cost.
Second, if either quench-hardening or tempering is simply removed from the heat treatment steps, any one of the necessary wear resistance (hardness), tensile strength and toughness is not ensured.